the petrogenesis of the 1.88–1.87 ga post- kinematic

1

The Petrogenesis of the 1.88–1.87 Ga post-

kinematic granitoids of the Central Finland

Granitoid Complex

by

Brent A. Elliott

Division of Geology and Mineralogy

Department of Geology

University of Helsinki

P.O. Box 64

FIN-00014 Helsinki, Finland

ACADEMIC DISSERTATION

To be presented, with the permission of the Faculty of Science of the University of Hel-

sinki, for public criticism in the small auditorium E204 of Physicum, Kumpula, on

May 23rd, 2001, at 12 o’clock noon.

Helsinki 2001

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Elliott, B.A., 2001. The Petrogenesis of the 1.88–1.87 Ga Post-kinematic Granitoids of the Cen-tral Finland Granitoid Complex. Academic dissertation, University of Helsinki, Finland. 36 pagesand 19 figures, with original articles (I–IV).

The evolution of the Svecofennian Orogen ranges from 1.95–1.80 Ga, with the main crust form-ing event occurring between 1.90–1.87 Ga. Subsequesnt deformation mainly occurred in southernFinland where it continued until ~1.80 Ga. A suite of post-kinematic 1.88–1.87 Ga plutons cross-cut 1.89–1.88 Ga synkinematic granitoids in the Central Finland Granitoid Complex (CFGC) insouth-central Finland. This post-kinematic suite is predominantly quartz monzonites and mon-zogranites and have been divided into three groups based on petrographic, mineral, and chemicalcharacteristics. Compared to the calc-alkaline synkinematic granitoids, the post-kinematic suitegenerally has an alkaline affinity with higher FeOtot, TiO2, K2O, Ba, Zr, and Nb and lower MgO,CaO and Sr at a given SiO2 content. The Type 1 plutons are peraluminous and Type 2 and 3 plu-tons are marginally metaluminous to peraluminous (A/CNK= ~1.0). The Type 2 plutons, espe-cially those in the western CFGC, approach the elevated incompatible element concentrations,tholeiitic and A-type geochemical characteristics of the 1.65–1.54 Ga rapakivi granites in south-ern Finland. The Type 3 plutons contain pyroxene-bearing phases (Type 3a) or pyroxenethroughout (Type 3b) and have many geochemical features that are associated with magmaticcharnockites. These plutons reflect high temperature relatively anhydrous magmas that began tocrystallize at pressures of 5-7 kbar at temperatures in excess of 825°C and were emplaced at pres-sures between 2-4 kbar. The redox conditions of the plutons were relatively reduced from –0.3to –1.5 log fO2 ∆FMQ, and record progressive oxidation and hydration during cooling. Radio-genic isotopes (Nd, Sr, Pb) of the post-kinematic plutons indicate homogeneous initial ratios: εNd(at 1875 Ma) ~0, 87Sr/86Sr ~0.703, and Stacey & Kramers-type Pb with low long-term Th/U ~2.Conventional and ion microprobe U-Pb data show that the post-kinematic magmatism took placeover quite a short time period (ca. 15 Ma) during overall lithospheric convergance and the mag-matism began in the northeastern CFGC at ~1.885 Ga and gradually moved to the west-southwestwhere the plutons are dated at ~1.870 Ga. The thickness of the crust, degree of partial meltingand proportion of lower crust and mantle derived mafic magma, subsequent fractionation, watercontent and rate of cooling during ascent all played important roles in the petrogenesis of thepost-kinematic suite. All of the post-kinematic plutons, despite mineralogical and geochemicalvariation, are thought to have been derived from similar parental sources and have undergonesimilar processes from generation to emplacement.

Key Words: granite, petrogenesis, Proterozoic, geochemistry, mineralogy, isotope geology, post-kinematic, Svecofennian orogeny, Central Finland Granitoid Complex, Finland

Brent A. Elliott, Department of Geology, P.O. Box 64, University of Helsinki, FIN-00014 Hel-sinki, Finland

ISBN 951-45-9992-6ISBN 951-45-9993-4 (PDF)

YLIOPISTOPAINOHELSINKI 2001

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CONTENTS

PREFACE …..………………………………………………………………………………. 4

INTRODUCTION ………………………...………………………………………………… 6

TECTONIC EVOLUTION OF THE CENTRAL FINLAND GRANITOID COMPLEX …. 6

THE CENTRAL FINLAND GRANITOID COMPLEX …..……………………………….. 8Geological background …...………………………………………………………… 8The 1.88–1.87 Ga post-kinematic granitoids of the CFGC…...…………………….. 9

The Type 1 granitoids…...………………………………………………….. 10The Type 2 granitoids…...………………………………………………….. 11The Type 3 granitoids…...………………………………………………….. 11

GEOCHEMISTRY…………………………………………………………………………… 12

MINERALOGY & MINERAL CHEMISTRY……………………………………………… 14Olivine …..………………………………………………………………………….. 14Pyroxene …...……………………………………………………………………….. 16Amphibole …...……………………………………………………………………... 16Biotite ...…………………………………………………………………………….. 18Fe-Ti Oxides …...…………………………………………………………………… 19Feldspars ……...…………………………………………………………………….. 19

INTENSIVE PARAMETERS …...………………………………………………………….. 20Pressure ……...……………………………………………………………………… 20Temperature …..…………………………………………………………………….. 20Oxygen Fugacity ……………………………………………………………………. 20

RADIOGENIC ISOTOPES …………………………………………………………………. 21

PETROGENESIS OF THE 1.88–1.87 GA POST-KINEMATIC GRANITOIDS …………. 22Protolith …………………………………………………………………………….. 22

Basalt ………………………………………………………………………. 24Diorite, Ferrodiorite, Ferromonzodiorite …………………………………... 24Mafic Granulite (lower crust) ……………………………………………… 24

Crystallization ………………………………………………………………………. 25

DISCUSSION ……………………………………………………………………………….. 29The structure of the crust in central Finland ………………………………………... 29The effect of water activity and cooling rate on magmatic evolution ……………… 30Temporal shift in magmatism across the CFGC …………………………………… 31

ACKNOWLEDGEMENTS …………………………………………………………………. 31

REFERENCES ……………………………………………………………………………… 32

PAPERS I–IV

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PREFACE

The thesis is based on the following four pa-pers, which are referred to in the text by theirRoman numeral notation:

I Elliott, B.A., Rämö, O.T. &Nironen, M. (1998). Mineral chemis-try constraints on the evolution of the1.88–1.87 Ga post-kinematic graniteplutons in the Central Finland Grani-toid Complex. Lithos 45, 109–129.

II Nironen, M., Elliott, B.A. & Rämö,O.T. (2000). 1.88–1.87 Ga post-kinematic intrusions of the CentralFinland Granitoid Complex: a shiftfrom C-type to A-type magmatismduring lithospheric convergance.Lithos 53, 37–58.

III Rämö, O.T., Vaasjoki, M., Mänt-täri, I., Elliott, B.A. & Nironen, M.Petrogenesis of the post-kinematicmagmatism of the Central FinlandGranitoid Complex I; Raiogenic iso-tope constraints and implications forcrustal evolution. Journal of Petrol-ogy, in press.

VI Elliott, B.A. Petrogenesis of the post-kinematic magmatism of the CentralFinland Granitoid Complex II; Proto-lith characteristics and magmatic evo-lution. Journal of Petrology, in re-view.

Paper IThis mineralogical study of the post-kinematic granitoids was the first paper detail-ing the mineralogical and petrographic fea-tures of the post-kinematic granitoids, dis-criminating the three types of plutons of the1.88–1.87 Ga suite. Mineral chemical datafrom feldspars, biotite, amphibole, pyroxenes,olivine and oxides were presented and inten-sive parameters (e.g. temperature, pressure,oxygen fugacity) were calculated from min-eral equilibria. The mineral equilibria suggesta range of pressures for crystallization; gener-ally 2–4 kbar for Al in Amphibole barometryand 5–7 kbar for those mineral assemblagesthat contain olivine and pyroxene or the ap-

propriate minerals for QUIlF (Quartz-Ulvöspinel-Ilmenite-Fayalite) equilibria.Temperatures mostly reflect subsolidus re-equilibration processes, recording tempera-tures during cooling of 450–800° C, but am-phibole-plagioclase thermometry and QUIlFcalculations estimate the magmatic tempera-tures above 800° C. Oxygen fugacity determi-nations indicate relatively reduced conditionsfor the formation of the post-kinematic grani-toids (the pyroxene-bearing assemblages inparticular) ranging between –0.3 and –1.5∆FMQ. Even though the plutons were em-placed during the orogenic event, post-datingregional compression, they have many min-eralogical and petrographic features similar tothe rapakivi granites of southern Finland.

B. A. Elliott’s contribution to the publicationincluded everything, with the exception of:the microprobe analyses; the collection ofmany of the rapakivi-related hand specimensfor petrography and associated microprobeanalyses used in the study.

Paper IIThis study provides a detailed description ofthe geochemistry of the post-kinematic grani-toids and the influence of tectonic setting onthe magma genesis of the 1.88–1.87 Ga suite.The Central Finland Granitoid Complexmainly consists of 1.89–1.88 Ga, highly de-formed, synkinematic, I-type granodiorite andgranite. The post-kinematic suite are slightlyfoliated to undeformed quartz monzonite togranite, and have been divided into three typesbased on petrographic, mineral and geochemi-cal characteristics. The post-kinematic suitegenerally has an alkaline affinity, with higherFeOtot, TiO2, K2O, Ba, Zr and Nb, and lowerMgO, CaO and Sr at a given SiO2 content.The Type 1 plutons are peraluminous and theType 2 and 3 plutons are marginally metalu-minous to marginally peraluminous (A/CNK=~1.0). The Type 2 plutons, especiallythose in the western CFGC, approach the ele-vated incompatible element concentrations,tholeiitic and A-type geochemical characteris-tics of the 1.65–1.54 Ga rapakivi granites insouthern Finland. The Type 3 plutons containpyroxene-bearing phases (Type 3a) or pyrox-ene throughout (Type 3b) and have many geo-chemical features that are associated with

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magmatic charnockites.

The generation of synkinematic granitoidsin the CFGC probably involved partial melt-ing of intermediate High-K rocks in the lowercrust, incorporating a mantle-derived compo-nent from the mafic underplate (or intraplate).The magmatic episode left a dry, granuliticresidue in the lower crust, which was partiallymelted to produce the post-kinemtic magmas.The post-kinematic magmatism took placeover a very short time of about ~15 Ma, whileconvergance was still continuing toward thesouth. This magmatism extended beyond theCFGC and registers a temporal shift fromnortheast to west-southwest, with a generalchange in geochemical character from C-typeto A-type affinity. The post-kinematic mag-mas were probably emplaced during extensionor transtension; the plutons with C-type affin-ity emplaced early in overthickened crust andthose plutons with A-type characteristicsslightly later when the crust was rather rigid.

B. A. Elliott’s contribution to the publicationincluded: field work and sample collecting;petrography and modal analyses; interpreta-tion and description of geochemical analysisresults for the CFGC granitoids; discussion ofthe post-kinematic granitoid magma-type andtemporal emplacement of the plutons acrossthe CFGC.

Paper IIINew U-Pb (conventional and ion probe), Nd,Pb, and Sr isotopic data for the post-kinematicplutons of the CFGC (both felsic and associ-ated mafic intrusions) and some of the 1.89–1.88 Ga synkinematic granitoids of the CFGCare presented here. This study, combined withpreviously published data, helps to constrainthe emplacement sequence of the post-kinemtic plutons throughout the CFGC andprovide some constraints on the source mate-rial that complements the petrogeneitc modelin Paper IV. Radiogenic isotopes (Nd, Sr, Pb)of the post-kinematic plutons indicate ho-mogenous initial ratios: εNd (at 1875 Ma) ~0,mantle-like initial 87Sr/86Sr ratios ~0.703, andStacey & Kramers-type Pb with low long-term Th/U ~2. Conventional and ion micro-probe U-Pb data show that the post-kinematicmagmatism took place over quite a short time

period (ca. 15 Ma) during overall lithosphericconvergance and the magmatism began in thenortheastern CFGC at ~1.885 Ga and gradu-ally moved to the west-southwest where theplutons are dated at ~1.870 Ga. The isotopiccomposition of the post-kinematic plutonswith that of the anorogenic rapakivi granitesof southern Finland are compared, allowingfor the identification of several Paleoprotero-zoic crustal terranes (microcontinents) thatwere amalgamated in the Proterozoic andform an essential part of the FennoscandianShield.

B. A. Elliott’s contribution to the publicationincluded: the discussion of the temporal evo-lution of the post-kinematic magmatism andthe isotopic characteristics of the post-kinematic granitoids across the CFGC; discus-sion of Svecofennian terannes in southernFinland; interpretation of the results of theisotopic study in conjunction with the petroge-netic model described in Paper IV.

Paper IVThis study incorporates the isotopic data fromPaper III and presents a geochemical model,constraining the nature of the source rock(s),the degree of partial melting, and theproportions of partial melts involved in thegenesis of the post-kinematic granitoids. The1.88–1.87 Ga post-kinematic granitoids of theCFGC provide a key geochemical link tounderstanding the processes involved ingranite formation in Paleoproterozoicorogenic and post-orogenic terrains. Threetypes of post-kinematic granitoid comprise aseries of plutons that migrate in age andgeochemical character from northeast tosouth-southwest across the CFGC, shortlyfollowing the peak of the Svecofennianorogeny. The thickness of the crust and intra-crustal differentiation processes played animportant role in the formation of the differentgranitoid types. In the eastern CFGC,pyroxene-bearing (Type 3) plutonspredominate and were formed from a mixtureof low to moderate degree partial melts (30–50%) of intermediate to mafic mantle-derived(ferrodioritic) source rocks and partial melts(10–30%) of mafic granulite lower crust atdepth. In the western CFGC, high-silica, iron-rich, fluorite-bearing (Type 2) plutons

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predominate. The higher thermal gradient,thinner upper and lower crust, andsignificantly more shallow Moho depthresulted in higher proportions of crustal meltsincorporated into the partial melts of a maficmantle-derived source. The geochemicalmodel, supported by isotopic data, suggeststhat despite the range of geochemicalcompositions and petrographic features, the1.88–1.87 Ga post-kinematic granites couldall have been derived from relatively similarsources, having undergone similarfractionation processes during their genesis, ashas been modeled for the Jämsä andHonkajoki plutons.

INTRODUCTION

The central part of the Fennoscandian Shieldin Finland is dominated by the CentralFinland Granitoid Complex (CFGC). TheCFGC encompasses an area of about 40,000km2 and consists mainly of granodiorite andgranite, but gabbroic, ultramafic, metavol-canic and metasedimentary rocks are alsofound throughout the complex (Paper II). TheCFGC hosts numerous 1.88–1.87 Ga post-kinematic granitoid intrusions that constrainthe age at which the crust was stabilized incentral Finland, and define a transition in ageand geochemical evolution from northeast towest-southwest. Mineral chemical studieswere carried out to constrain the conditions offormation and emplacement of the post-kinematic granitoids. Detailed mapping andsampling ensued in order to gain a better per-spective of the distribution of the post-kinematic plutons throughout the CFGC. Amore detailed whole-rock and isotope geo-chemistry was made to define the magmaticaffinities and source constraints of the post-kinematic granitoids, and to compare with thesynkinematic granitoids of the CFGC andanorogenic granites of southern Finland. Acomparison of whole rock major- and trace-element geochemistry between bimodal,multi-phase type intrusions in the westernCFGC and pyroxene-bearing intrusions lo-cated mainly in the eastern CFGC provide in-sights into the petrogenesis of this late stagegranitic magmatism that shortly followed theorogenic culmination in an anomalously thick

lithosphere. The primary goal of this thesis isto incorporate mineral chemistry, whole-rockgeochemistry, and isotope geochemistry into adetailed petrogenetic model constraining thesource(s) and processes that were involved inthe formation of the 1.88–1.87 Ga post-kinematic plutons of the CFGC.

TECTONIC EVOLUTION OF THECENTRAL FINLAND GRANITOIDCOMPLEX

The Svecofennian Orogen has been dividedinto three tectonic units based on lithologicalcriteria and U-Pb zircon age data. In centraland southern Finland these units form a primi-tive arc complex adjacent to the Archean cra-ton, the accretionary complex of central andwestern Finland, and the southern Finland ac-cretionary complex (see Figure 1; cf. Kors-man et al., 1997; Nironen, 1997; Korsman etal., 1999; Rämö et al. 1999).

The accretionary arc of southern Finlandcontains the volcanic-sedimentary Uusimaaand Häme Belts. The Uusimaa belt is charac-terized by greywacke with volcanic and car-bonate rock intercalations, and a belt of bi-modal volcanic rocks. The mafic volcanicrocks with MORB affinities (together withcarbonate rocks) were probably deposited on aPaleoproterozoic ensialic crust (Lindroos &Ehlers, 1994). The volcanic rocks of theHäme Belt are characterized by the presenceof 1.89 Ga intermediate island-arc type rocksto mafic rocks with increasing within plateaffinity. The southern Finland arc containsmigmatites with 1.89–1.88 Ga trondhjemiticto granodioritic leucosomes formed from agreywacke host rock and 1.84–1.82 Ga granitemigmatites associated with the late-orogenicmagmatism (Korsman et al., 1997). Late oro-genic rocks are S-type granites with agesaround 1.84–1.82 Ga, and are predominantlyfound in a broad belt trending northeast-southwest across southern Finland. The post-orogenic rocks, dated at 1.81–1.79 Ga, havealkalic/shoshonitic affinities and range incomposition from quartz monzonite to granite.They are found as small intrusions close to thenorthern margin of the 1.84–1.82 Ga belt oflate-orogenic granites. The last major episode

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of magmatism of the Svecofennian domain isthe 1.65–1.54 Ga anorogenic A-type rapakivigranites that are temporally and spatially asso-ciated with mafic rocks (gabbroic and anor-thositic rocks, diabase dikes) and were em-placed in an extensional tectonic regime (seeHaapala & Rämö, 1990; 1992; Korsman et al.,1999).

The accretionary complex of central andwestern Finland contains the CFGC, the Both-nia Belt to the west of the CFGC and theTampere Belt and metasedimentary PirkkalaBelt to the south of the CFGC. The TampereBelt is composed of well-preserved volcanicand sedimentary rocks ranging from >1.91 Gamafic MORB-type volcanics (Kähkönen &Nironen, 1994) to younger, 1.90–1.89 Ga,fore-arc greywacke and andesitic volcanicrocks (Kähkönen et al., 1989). The PirkkalaBelt is characterized by 1.885 Gathrondhjemitic and granodioritic migmatitesformed from psammitic greywacke host rocks.Small ultramafic intrusions and mafic MORB-type volcanic rocks, and change in migmatiteleucosome character, have been interpreted todelineate a suture between the Pirkkala Belt ofthe central Finland arc and the Häme Belt ofthe southern Finland arc (cf. Lahtinen, 1994;Rämö et al. 1999).

Lahtinen (1994) presented a cross-sectional evolutionary model for the Sve-cofennian of Finland. The model involves theclosure of the marine basin and collision oftwo arc systems 1.89 Ga ago. Nironen (1997)extended the evolutionary model to the entireSvecofennian crust (including Sweden, Esto-nia, and Lithuania) and presented it in a two-dimensional form (Figure 2).

The evolutionary model of Nironen(1997) entails:

(A) Initial accretion of an island arc com-plex (the central Finland arc) to theArchean continent took place 1.91 Gaago;

(B) followed by subduction reversal androtation of plate motion direction, orconvergence of another (micro-)plate.

(C-D) The subsequent collision of anotherarc complex (the southern Finlandarc) against the Central Finland arc,leading to the disappearance of twosubduction zones with opposite dips,and the merging of two accretionaryprisms.

According to the tectonic model of Ni-ronen (1997), an extension of the Lahtinen(1994) model, collision of the arc complex ofthe central Finland arc to the Archean craton,and the subsequent collision of the southernFinland arc to the central Finland arc at 1.91–1.89 Ga caused lithospheric thickening in theCFGC area. Part of the overthickened litho-spheric mantle was delaminated and compen-sated by hot asthenosphere. Decompressionmelting of the asthenosphere produced mantlemagmas that rose to the base of the crust andinduced melting in the lower crust. The maficunderplate and intraplate (i.e. the high veloc-ity lower crust) induced partial melting of K-rich rocks of intermediate composition in thelower crust with additional mantle-derivedmaterial. The result was the production of thesynkinematic magmatism in the CFGC, leav-ing an infertile granulite residue in the lowercrust. Partial melting of the granulitic residueand mantle-derived material produced the1.88–1.87 Ga post-kinematic magmatism,which was emplaced in an extensional or tran-spressional environement at a time when con-vergance was still occurring to the south in thesouthern Finland arc.

THE CENTRAL FINLAND GRANI-TOID COMPLEX

Geological background

The supracrustal rocks of the Svecofennianorogen are mainly turbiditic sediments thatwere metamorphosed at amphibolite faciesconditions. Metavolcanic rocks, occurring innarrow 1.90–1.88 Ga age belts, are mostly in-termediate in composition and have arc-typegeochemical affinities. The MORB-type ma-fic volcanic rocks form minor units and havegenerally been thought to represent the lowestparts of the supracrustal sequences.

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Figure 2. Tectonic model of the Svecofennian Orogen (after Nironen, 1997). Stippled pattern denotes Archeancrust; the area without overprint is oceanic crust and sedimentary rocks; the grey areas are arc complexes, andthe dotted areas are volcanic arcs.

1.90 Ga

?

500 km

B)

1.91 Ga500 km

A)

?

Ophiolites

1.88 Ga

500 km

C)

1.87 Ga

500 km

D)

arc

complex

volcanic

arc

Deep seismic sounding studies (Luosto etal., 1990; Korja, 1991; Luosto, 1991) haveshown that the crustal thickness in the south-ern part of the Fennoscandian Shield variesconsiderably, from 40 to 65 km. The crust isthickest, at ~65 km, in eastern Finland alongthe contact with the Svecofennian Orogen andKarelian Domain.

The synorogenic plutonic rocks range incomposition from gabbro to granite, with I-type granodiorite as the prevailing rock type(see Nurmi & Haapala, 1986). The oldestrocks of this group are the 1.93–1.92 Ga to-nalitic gneisses, but crystallization ages aregenerally 1.89–1.88 Ga (Vaasjoki, 1996).There are a few age dates of the syn-kinematic

granitoids in the CFGC, and these have alsoyielded ages around 1.89–1.88 Ga.

The 1.88–1.87 Ga post-kinematic granitoidsof the CFGC

Besides the 1.89–1.88 Ga rocks, the CFGCcomprises a distinct suite of granites thatcrosscut the 1.89–1.88 Ga granitoids and yieldslightly younger ages around 1.88–1.87 Ga.As these granites were emplaced after majorcontractional movements related to collision,they are considered post-kinematic. The post-kinematic granites are found as at least 24stocks and small (<500 km2) batholiths (seeFigure 3). They are weakly to non-foliated,medium- to coarse-grained granites (many ofwhich have multiple phases) that are, on aver-

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diorite or monzogranite and plagioclase andalkali-feldspar are resorbed and largely alteredto sericite. Plagioclase is often surrounded orreplaced by secondary alkali feldspar. Alkalifeldspar occurs as perthitic orthoclasemegacrysts and medium-grained microcline.Myrmekitic intergrowths are uncommon, butwhen present, pervasive. Quartz is typicallyinterstitial and forms late stage inclusions andsmall composite veins.

Biotite is usually the only mafic silicate,with the exception of hornblende-biotite plu-ton at Lavia, and appear as (1) reddish brown,anhedral grains that commonly contain zircongrains with radioactive haloes, and (2) brownto light-brown, euhedral to subhedral grains.The reddish brown biotite may be a reactionproduct of amphibole, which probably pre-ceded the euhedral biotite in the sequence ofcrystallization. Accessory minerals include

Figure 3. Simplified geologic map of the Central Finland Granitoid Complex showing the names and locationsof the post-kinematic granitoid plutons.

age, more potassium-rich and have higher Fe/Mg than the syn-tectonic granitoids (Nironen& Rämö, 1995; Lahtinen, 1996). The post-kinematic granites may be divided into threegroups based on petrography, mineralogy,outcrop characteristics, mineral chemical andgeochemical variation.

The Type 1 granitoidsType 1 plutons flank the CFGC in the south,are discordant, slightly to non-foliated andconsist of even- to coarse-grained, often por-phyritic, biotite granodiorite or granite. Com-pared to the Type 2 and 3 plutons, they tend tocontain more modal plagioclase, appear white/gray to pink in color, and show distinctlylower Fe/Mg than the Type 2 and 3 post-kinematic plutons (Paper I; Paper II; Lahtinen,1996).

The Type 1 plutons are typically grano-

100 km

Kurikka

Luopa

Type 1

Type 2

Type 3

HonkajokiParkano

Karkku

Siitama

JuupajokiJämsä

Vekkula

Kaipola

Petäjävesi

Rautalampi

Puula

Muurame

Lavia

Suontee

Paloinen

Saarijärvi

Lahnanen

Suolahti

Kihniö

Nummijärvi

Orivesi

Konnevesi

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anhedral, interstitial fluorite, rounded zonedzircon, and euhedral apatite, and ilmenite asthe sole Fe-Ti oxide.

The Type 2 granitoidsThe Type 2 plutons are concentrated in thewestern and southern CFGC and are the mostabundant type of pluton in the post-kinematicsuite. They are discordant, usually non-foliated, even- to coarse-grained, often por-phyritic, red granite. The plutons are margin-ally peraluminous and show high Fe/Mg thatincreases with increasing silica (Paper II;Lahtinen, 1996). Several of the Type 2 plu-tons are associated with coeval mafic plutonicrocks (Rämö, 1986; Rämö et al., 1999).

The main mafic silicate is biotite, butsome plutons include one or more maficphases that are dominated by amphibole. Bio-tite occurs as a primary mineral phase, and asa reaction product associated with the marginsof many amphibole grains. Amphibole is an-hedral, pleochroic and green-brown in color,and shows some reaction to biotite on grainmargins. The mafic silicates are generally in-terstitial, but some euhedral biotite grains areenclosed in feldspar grains. Plagioclase islargely altered to sericite (sausseritisized) andcommonly replaced by alkali feldspar. Myr-mekitic intergrowths are common and a com-posite mantle of plagioclase and quartz occa-sionally surrounds large orthoclasemegacrysts. Quartz is interstitial and oftenforms roundish inclusions in feldspars. Acces-sory minerals include fluorite, zircon, apatite,titanite, and allanite. Zircon is euhedral tosubhedral, zoned, often rhombic and enclosedin biotite and amphibole. Fluorite and apatiteare ubiquitous; apatite forming small, eu-hedral, acicular to prismatic grains, and fluo-rite as anhedral interstitial grains. Epidotemay also occur and is associated with altera-tion of allanite. Titanite often forms a reactionrim around ilmenite and is commonly en-closed in biotite or amphibole grains.

The Type 3 granitoidsType 3 plutons, generally situated in the east-ern part of the CFGC, are pyroxene bearing,typically discordant and consist of even- tocoarse-grained, often porphyritic, non-foliated, quartz-monzonite and granite. They

are divided into two subgroups: Those with apyroxene-bearing margin or small pyroxene-bearing bodies located near the margin (Type3a), and those with pyroxene throughout(Type 3b; cf. Paper II). The contacts betweenthe marginal assemblage and the center of thebody are gradual and marked by a steep de-crease in magnetic susceptibility. These plu-tons show a decrease in Ti, Fe/Mg and K/Nafrom the marginal assemblage toward the cen-ter of the pluton (Paper I).

The Type 3 post-kinematic plutons typi-cally consist of a pyroxene±olivine-bearingquartz monzonite margin, which grades intobiotite-hornblende monzogranite at the center.Plagioclase is unaltered and appears to repre-sent a late cumulus phase. Grain boundariesare typically resorbed and occasionally inter-locking. Alkali feldspar occurs as largeperthitic orthoclase megacrysts, often zonedand in places mantled by composite quartzand plagioclase.

The marginal assemblage of these plutonscontains fayalitic olivine, Fe-rich clinopyrox-ene and or Fe-rich orthopyroxene, amphiboleand biotite; olivine, and pyroxene are equallyabundant, but less than amphibole. Amphiboleforms large, pleochroic green to brown-green,rarely blue-green, interstitial grains, as well ascoronas around some olivine and pyroxenegrains. Biotite typically forms large lightbrown, subhedral grains, but can also occur asa secondary reaction product of amphibole.Orthopyroxene (ferrosilite) occurs in close as-sociation with olivine and clinopyroxene. Cli-nopyroxene (ferroaugite) occasionally con-tains pigeonite exsolution lamellae. Myrme-kitic intergrowths between alkali feldspar andplagioclase are common.

In the plutons that do not contain pyrox-ene(s) throughout, but only as a marginal as-semblage, the change from the marginal as-semblage to the central unit is marked by theabsence of olivine and pyroxene, and a de-crease in magnetic susceptibility due to thelack of magnetite formed during the reactionof olivine and pyroxene to amphibole. Amphi-bole is occasionally replaced by biotite, whichbecomes more abundant toward the center ofthe pluton. Accessory minerals include zircon,

12

apatite, and secondary titanite.

GEOCHEMISTRY

In Harker diagrams, the post-kinematic grani-toids display relatively coherent trends thatgenerally plot between the trends of syn-kinematic granitoids from the CFGC and therapakivi granites of southern Finland (see Pa-per II). In the Al2O3 vs. SiO2 diagram, thepost-kinematic and synkinematic intrusions ofthe CFGC show rather similar decreasingtrends with increasing SiO2 (Figure 4a). TheFeOtot (and TiO2) values of the post-kinematicsuite are somewhat higher, and MgO and CaOcontents are lower than those of the syn-kinematic granitoids at a given SiO2 content(Figure 4b, c, & e). The Type 3a plutons havelower MgO and higher FeOtot than the Type3b plutons at similar SiO2 contents. The Type2 and 3 plutons have elevated FeOtot/MgO ra-tios compared to the Type 1 plutons and thesynkinematic granitoids, although the syn-kinematic suite has fairly high FeOtot/MgO.The FeOtot/MgO ratios of the Type 3a plutonsdecrease from the margins (SiO2 less than64%) towards the center. In the A/CNK vs.FeO*/(FeO*+MgO) diagram, the Type 1 plu-tons are clearly peraluminous whereas theType 2 and 3 plutons show trends from mar-ginally metaluminous to marginally peralumi-nous (Figure 4f). In this respect, the post-kinematic plutons are more similar to typicalrapakivi granites than to the syn-kinematicgranitoids (cf. Haapala & Rämö, 1992).

The Type 1 post-kinematic intrusions arelower in Na2O than the synkinematic grani-toids and the rapakivi granites. In the K2O vs.SiO2 diagram, the syn-kinematic granitoidsdefine a weakly ascending trend and generallyplot in the high-K calc-alkaline field. Thepost-kinematic granitoids have even higherK2O contents and plot partly in the sho-shonitic field. The Lahnanen pluton is distinctfrom the other Type 3 plutons with higherSiO2 (around 72%) and relatively low K2Oand Rb contents. Nironen & Front (1992) sug-gested that K2O and Rb were enriched in re-sidual melts that subsequently crystallized inpockets and shear zones rich in garnet and or-thoclase within the Lahnanen pluton. This is

probably the reason for low abundance ofthese elements in the pyroxene granodiorite.The Type 3b Rautalampi pluton has the low-est SiO2 contents of all the post-kinematic plu-tons, and is distinctive on most Harker dia-grams, plotting within the synkinematic trend.Even though the pluton has been included inthe post-kinematic group, based on petrogra-phy and its relatively undeformed state, thegeochemistry seems to suggest that it may bea synkinematic granitoid emplaced late and/orin an area of transpressive stress of extension.

The Ba and Zr (also Nb) values are gener-ally higher and Sr values lower in the post-kinematic plutons than in the synkinematicgranitoids at similar SiO2 contents. The Rb/Baratios of the post-kinematic plutons show anincreasing trend, contrary to the almost con-stant Ba concentrations with increasing Rb ofthe synkinematic rocks (Figure 5b). Thesynkinematic granitoids generally have higherSr concentrations and distinctively higher Sr/Ba ratios when compared to the post-kinematic granitoids (Figure 5b). Concentra-tions of F are slightly higher in the Types 1and 2 plutons than in the Type 3 plutons andthe synkinematic granitoids. In particular, thehigh-silica varieties of the Type 2 granites inthe western CFGC approach the elevated F,Rb, Nb, and Y values of the rapakivi granites(cf. Paper II).

The REE patterns of the post-kinematicgranitoids are grossly similar, with mean (La/Yb)N ratios between 11 and 12. The Type 1and 2 plutons have negative Eu anomalies(mean Eu/Eu* 0.49 and 0.37, respectively;Figure 6a, b). The Type 3 plutons generallyhave negative Eu anomalies but the marginalassemblages have rather straight normalizedpatterns or show slightly positive Eu anoma-lies (Figure 6c, d). For example, the Type 3aJämsä pluton shows clear decrease in Eu/Eu*from the pyroxene-bearing margin to the cen-ter of the pluton.

The REE patterns of the post-kinematicplutons are similar to those of the syn-kinematic granitoids but the REE contents arelower in the latter. All but the one sample ofthe synkinematic granitoids have negative Euanomalies (mean Eu/Eu* = 0.59). The REE

13

Figure 4. Harker diagrams of Al2O3, MgO, FeO*, K2O, and CaO vs. SiO2 and molecular A/CNK vs. FeO*/(FeO*+MgO) for post kinematic and synkinematic granitoids from the Central Finland Granitoid Complex.Boundaries for the subdivision of subalkalic rocks of Peccerillo & Taylor (1976), Ewart (1982), Innocenti et al.(1982), Carr (1985) and Middlemost (1985) as summarized by Rickwood (1989) are shown as dashed lines inFigure 4d. Post-kinematic granitoid symbols are: Type 1 - white circles; Type 2 - grey circles; Type 3a - greycircles with thick black outlines; Type 3b - solid black circles; and synkinematic granitoids are small black dots.

SiO2

SiO2

SiO2

SiO2

SiO2

50 55 60 65 70 75 80

AlO2

3

11

12

13

14

15

16

17

18

19

50 55 60 65 70 75 80

K

O2

1

2

3

4

5

6

7

50 55 60 65 70 75 80

MgO

0

1

2

3

4

5

50 55 60 65 70 75 80

CaO

0

2

4

6

8

50 55 60 65 70 75 80

FeO*

0

2

4

6

8

10

12

A/CNK

0.8 0.9 1.0 1.1 1.2 1.3 1.4

FeO*/(FeO*+MgO)

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

PeraluminousMetaluminous

Shoshonitic

High-K calc-alkaline

Calc-alkaline

A)

B)

C)

D)

E)

F)

14

patterns for rapakivi granites differ remarka-bly from both groups, having a strongly nega-tive Eu anomaly (mean Eu/Eu* = 0.25) andlower (La/Yb)N ratios.

The post-kinematic and synkinematicgranitoids generally plot in the same area ontectonomagmatic diagrams (Figure 7 a, b).The synkinematic granitoids fall more withinthe volcanic arc and syn-collisional granitesfield on Nb vs. Y, and in volcanic-arc granitesfield on Rb vs. Y+Nb. The post-kinematicgranitoids tend to plot more within the within-plate granite field, but the Type 3 plutonsmore towards the volcanic-arc granites field inthe Rb vs. Y+Nb discrimination diagram. Al-most all post-kinematic and synkinematic

samples plot in the area designated by Pearce(1996) as the post-collisional field (cf. PaperII). These tectonomagmatic diagrams do notreally confirm the tectonic setting, but identifythe transitional nature from a volcanic arc orsyn-collisioanl type environment toward amore stable cratonic setting for which thegranites were emplaced.

MINERALOGY & MINERAL CHEM-ISTRY

OlivineOlivine is fayalite and only found in thequartz monzonite and monzogranite thatforms the marginal assemblages of several

Figure 5. Scatterplot diagrams of Sr and Rb vs. Ba. Symbols are the same as indicated in Figure 4.

0 1000 2000 3000 4000 50000

50

100

150

200

250

300

0 1000 2000 3000 4000 5000

0

200

400

600

800

1000

1200

A)

B)

Rb (ppm)

Ba (ppm)

Ba (ppm)

Sr (ppm)

15

Figure 6. Chondrite normalized (Boynton, 1984) Rare Earth Element (REE) plots for (A) Type 1 post-kinematic granitoids; (B) Type 2 post-kinematic granitoids; (C) Type 3apost-kinematic granitoids; and (D) Type 3b post-kinematic granitoids. The synkinematic granitoids are shown as light grey REE plots in each diagram for comparison.

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple

(cho

ndrit

eno

rmal

ized

)

1

10

100


Sam

ple

(cho

ndrit

eno

rmal

ized

)

1

10

100


Sam

ple

(cho

ndrit

eno

rmal

ized

)

1

10

100


Sam

ple

(cho

ndrit

eno

rmal

ized

)

1

10

100

A)

B)

C)

D)

16

Type 3 plutons. The composition is relativelyhomogenous, ranging from Fa92 to Fa94.

PyroxeneOrthopyroxene is ferrosilite and clinopyrox-ene is ferroaugite (Figure 8). One pyroxene orboth pyroxenes may be encountered in thequart monzonite and monzogranites of theType 3 plutons. The ferrosilite has Fe/(Fe+Mg) ratios ranging from 0.75 to 0.86, andin ferroaugite ranging from 0.83 to 0.84. Octa-hedral Al3+ (<0.03) and Ti4+ (<0.03) are rela-

tively constant for all the pyroxenes. The or-thopyroxene grains in the Rautalampi plutonhave consistently lower Fe2+/(Fe2++Mg) ratios(0.74–0.77), but overall pyroxene mineralchemical compositions are homogeneous.

AmphiboleAmphibole has only been observed in theLavia pluton in the Type 1 plutons, and isgenerally absent as a major ferromagnesiansilicate in the biotite dominated Type 2 plu-tons. Type 3 plutons that contain a pyrox-

Figure 7. Discrimination diagrams for tectonic affinity of granites (A) Nb vs. Y (Pearce et al., 1984); and (B) Rbvs. Y+Nb (Pearce, 1996). Symbols are the same as indicated in Figure 4.

1 10 100 10001

10

100

1000

Within-plate granites

Volcanic-arc

and

syn-collisional granites

Ocean ridge granites

Within-plate granitesSyn-collisional granites

Volcanic-arc granites

Ocean ridge granites

1 10 100 10001

10

100

1000

A)

B)

Nb

(p

pm

)R

b (

pp

m)

Y (ppm)

Y+Nb (ppm)

17

Figure 8. Enstatite–Diopside–Hedenbergite–Ferrosilite quadralateral diagram for the classification of pyrox-enes. Grey circles with thick black outlines represent pyroxene from Type 3a post-kinematic plutons, and solidblack circles indicate pyroxene from Type 3b post-kinematic plutons.

Figure 9. (A) Mg/(Mg+Fe2+) and (B) Fe3+/(Fe3++AlVI) vs Si in formula unit diagrams for the classification amphi-bole (Leake, 1997). Symbols are the same as indicated in Figure 4, except no amphibole from Type 1 plutonshave been analyzed to date.

Fs

Hd

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

En

Di50

60

70

80

90

100

1.0

0.80

0.60

0.40

0.20

0.007.50 7.00 6.50 6.00 5.50

Ferro-edenite

Ferropargasite

Si in formula

Fe

/(F

e+

Al

)3+

3+V

I

Hastingsite

B)

0.50

0.40

0.30

0.20

0.10

0.007.50 7.00 6.50 6.00 5.50

Ferro-edenite

Mg/

(Mg+

Fe

)2+

A)

Hastingsite (Al <Fe )or

Ferropargasite (Al Fe )

VI 3+

VI 3+≥

18

ene±olivine-bearing assemblage always con-tain substantial amounts of amphibole. Theiramphibole content decreases from the margintowards the center of the plutons, when themarginal phase is present, otherwise remain-ing relatively constant. Amphibole range incomposition from ferro-edentic hornblende tohastingsitic hornblende (or ferro-edenite andhastingsite under the Leake (1997) classifica-tion) and are relatively iron-rich with Fe2+/(Fe2++Mg) ratios ranging from 0.74 to 0.89(Figure 9).

Amphibole in the Type 3 plutons showremarkable compositional variation from mar-gin toward the center of the pluton. Exchangereactions involving Fe, Ti, and Mg in the C-site show a distinct decrease in Fe and Ti andan increase in Mg. While the overall whole

rock Fe/Mg ratio increases, the Fe/Mg ratio inamphibole decreases toward the center. Ex-change reactions of Na and K, Ca show littlevariation. Amphibole compositions tend toincrease in Na and become slightly more cal-cic toward the center. In general, coupled sub-stitutions are strongly influenced by silica ac-tivity and exchange reactions involving Fe,Mg, and Ti. Amphibole compositions at themargin have higher Fe2+, Fe3+, Ti, Na, K, andAltot, and the compositions are progressivelyenriched in Mg, Ca, and Si toward the center.

BiotiteBiotite is the most common ferromagnesiansilicate in the post-kinematic plutons. It isfound as a primary subhedral to anhedralgrains, typically interstitial, but may formsubophitic-like textures with feldspars. Secon-

Figure 10. The Mg–AlVI+Fe3++Ti–Fe2++Mn ternary diagram for the classification of trioctahedral micas (afterFoster, 1963). Mg and Fe2+ fields correspond to Mg– and Fe-rich micas, respectively. Symbols are the same asindicated in Figure 4.

Fe 2++Mn0 10 20 30 40 50 60 70 80 90 100

Al VI +Fe 3++Ti

0

30

40

50

60

70

80

90

100

Mg

0

10

20

30

40

50

60

70

80

90

100

Mg

Fe2+

Anniteand

Siderophyllite

10

20

19

dary biotite is found as reaction productaround amphibole, pyroxenes, and rarely oli-vine. The biotite is relatively iron-rich withFe2+/(Fe2++Mg) ranging from 0.61 to 0.79(Type 1 plutons), 0.72 to 0.94 (Type 2 plu-tons), and 0.77 to 0.88 (Type 3 plutons). Bio-tite shows very little variation in tetrahedralAluminum; 2.17 to 2.35 (Type 1), 1.87 to 2.21(Type 2), and 2.11 to 2.33 (Type 3) (AlIV at-oms per 24 oxygens). On the AlVI+Fe3++Ti-Fe2++Mn-Mg ternary diagram, all the post-kinematic granite biotites are relatively ironrich, and the Type 1 granite biotites are dis-criminated by slightly higher Mg, Al, and Ticontents (Figure 10).

Fe-Ti OxidesIlmenite is the predominant Fe-Ti oxide in allthe post-kinematic plutons. Magnetite coexist-ing with ilmenite is only found in the pyrox-ene±olivine bearing marginal assemblages ofthe Type 3 plutons. Hematite and more Mn-

Figure 11. Albite–Orthoclase–Anorthite ternary digram for the classification of plagioclase and alkali feldspar.Symbols are the same as indicated in Figure 4.

rich oxides have been observed from the Type2 Honkajoki and Parkano plutons, indicativeof higher oxygen fugacities and/or subsolidusalteration. The composition of ilmenite andmagnetite is relatively homogeneous rangingfrom 0.94 to 0.98 XILM, and 0.97 to 0.99 XMT.

FeldsparsAlkali feldspar occurs as coarse perthitemegacrysts and smaller microcline grains.Plagioclase is more prevalent in the olivine-and pyroxene-bearing plutons and anorthitecontents tend to vary slightly between plutontypes of the post-kinematic suite. The compo-sitional range of most alkali feldspar in thepost-kinematic plutons is Or82–99 (Figure 11).The composition of plagioclase in Type 1 plu-tons is oligoclase to andesine (An18–41), theType 2 plutons predominantly oligoclase(An11–28), and rarely albite, and in the Type 3plutons oligoclase to andesine (An18–42).

An0 10 20 30 40 50 60 70 80 90 100

Or

0

10

20

30

40

50

60

70

80

90

100

Ab

0

10

20

30

40

50

60

70

80

90

100Albite

OligoclaseAndesine Labradorite Bytownite Anorthite

Orth

ocla

se

Ano

rthoc

lase

20

INTENSIVE PARAMETERS

PressureThe composition of amphibole and olivine-pyroxene equilibria were used for pressuredetermination. Estimated pressures of crystal-lization for amphibole, using the Al in amphi-bole geobarometer proposed by Hammerstrom& Zen (1986), are fairly consistent in therange of 2.4 to 4.3 kbar. Mineral reactions inthe plutons that contain Fe-rich olivine andpyroxene-bearing assemblages are considera-bly more pressure sensitive (Smith, 1971,1974; Lindsley, 1983; Bohlen & Essene,1978; Koch-Müller et al, 1992). A projectionof the pyroxene compositions, and the experi-mentally derived data for Fe-rich olivine-pyroxene-quartz assemblages (Smith 1971,1974) indicate maximum pressures of 7.5 to8.0 kbar. Similarly, calculations using themodel calculations of Koch-Müller et al. esti-mate pressures averaging 8.9 kbar, assuming amaximum temperature of 900°C. Applicationof the QUIlF program to the olivine-pyroxeneassemblages provides pressures closer tothose calculated with the amphibole barome-ter, ranging from 3.2 to 8.2 kbar. There isquite a range in the calculated pressures, aver-aging 2.0 to 4.0 kbar (Al in amphibole) to 5.0to 7.0 kbar (olivine-pyroxene QUIlF equilib-ria).

TemperatureThe two-feldspar geothermometry applica-tions (Fuhrman & Lindsley, 1988; Green &Usdansky, 1986; Stormer, 1975) yielded tem-peratures ranging from 380°C to 650°C, andthus failed to provide reasonable magmatictemperatures; those obtained are mostly re-garded as subsolidus re-equilibration esti-mates. Biotite stability curves (Wones & Eug-ster, 1965) record maximum temperatures inthe range of 700°C to 875°C for Type 3 plu-tons, and temperatures between 820°C and950°C for Type 2 plutons. Average tempera-ture estimates for amphibole, using the plagio-clase-amphibole geothermometer proposed byBlundy & Holland (1990), gave consistent es-timates between 756°C and 806°C. Their sub-sequent thermometer (Holland & Blundy,1994) yielded a wider range of temperatures,452°C to 875°C. The QUIlF assemblages (cf.

Anderson et al., 1993) of the post-kinematicsuite recorded a temperature range of 450°Cto 800°C with lower overall temperatures foroxide-oxide calculations. Zircon saturationthermometry (Watson & Harrison, 1983) indi-cate a wide range of temperatures between950° to 800° C for wt. % SiO2 compositionsaround 55 and 75, respectively. The tempera-ture estimates recorded by the biotite stabilitycurves and amphibole appear to be magmatic,but the two-oxide and QUIlF thermometersreflect a re-equilibration cooling trend. Whereall three thermometers could be utilized, thetemperature estimates concur with field obser-vations and cooling trends of individual plu-tons (Elliott, 1997).

Oxygen FugacityOxygen fugacities calculated for biotite-ilmenite stability curves range from -1.6 to0.65 ∆FMQ log fO2, probably reflecting thedisassociation of gases with the crystallizationof hydrous mafic silicate minerals. Oxygenfugacities calculated by the QUIlF programfor olivine-pyroxene bearing assemblages arerelatively more reduced, ranging between -0.3and -1.7 ∆FMQ fO2. It has been suggested thatthe production of hornblende or biotite duringthe evolution of a magma should increaseoxygen fugacity (Frost, 1990). During theevolution of the magma, an increase in the ac-tivity of H2O will cause the consumption ofmagnetite, augite, and quartz, production ofhornblende, and liberation of oxygen accord-ing to HMAQ equilibrium (Hornblende +Oxygen = Augite + Magnetite + Quartz+Fluid).

Most of the plutons of the post-kinematicsuite contain ilmenite as the only Fe-Ti oxideand biotite as the only mafic silicate and ap-pear to have been controlled by the KUIlBequilibrium (Biotite + Ilmenite = Feldspar +Ulvöspinel + Fluid).

The olivine-pyroxene assemblages gener-ally co-exist with magnetite and ilmenite andwere controlled by the QUIlF equilibrium(Quartz + Ulvöspinel = Ilmenite + Fayalite).

In the Type 3 plutons, relatively low oxy-gen fugacity condition (below FMQ) allowediron to be incorporated into mafic silicates,

21

but during disassociation and the formation ofa fluid phase, oxygen fugacity increased caus-ing iron to be distributed preferentially intoFe-Ti oxides (i.e. ilmenite). Moving from theolivine- and pyroxene-bearing marginalphases to the hornblende-dominated interiorof these plutons, Fe/Mg ratio and Ti abun-dance in the mafic silicates decrease, oppositeto the behavior of other petrogentic indicatorsof differentiation, such as SiO2 and Fe/Mg inthe whole-rock (Paper I). This can be ex-plained by the increasing activity of the fluidphase in the KUIlB reaction resulting in a pro-gressively increasing oxidation state of theevolving residual magma from which the cen-tral parts of the pluton eventually crystallized.

RADIOGENIC ISOTOPES

A detailed description of U-Pb (conventional

and ion probe), Nd, Pb, and Sr isotopic data isreported in Paper III and offers insights intothe timing of granitoid emplacement andsource character of the post-kinematic grani-toids of the CFGC. The U-Pb dating shows aslight, yet significant, shift in age of the plu-tonism across the CFGC. The plutons in thenortheast and east-central part of the CFGCgive U-Pb ages around 1885–1880 Ma, whilethe plutons in the southern and western partsof the CFGC record younger ages between1880–1870 Ma. The U-Pb geochronologicaldata constrains the temporal evolution of thepost-kinematic magmatism in the CFGC andshows a shift in geochemical composition inthe post-kinematic suite with time. Initial εNdvalues are near chondritic (at 1875 Ma) andshow no clear differences between the threetypes of post-kinematic plutons (Figure 12).Low overall Rb/Sr in the granitoids of theJämsä and Honkajoki plutons are indicated by

Figure 12. Simplified map of the Central Finland Granitoid Complex and location of the post-kinematic plutonsshowing the initial εεεεNd (at 1875 Ma) values of the post-kinematic suite and some values for the synkinematicrocks (indicated by small black dots). Nd data from Appendix Paper 3, Huhma (1986), Patchett & Kouvo(1986), Makkonen (1996), Lahtinen & Huhma (1997), and Rämö & Nironen (in preparation). Pluton name andtype are as indicated in Figure 3.

100 km

-0.7

-0.5

-0.4-0.9

-1.1

-0.1-0.4-0.6

+0.4

-0.1+0.2+0.3+0.5

+0.1

-0.6

-0.1

-1.0

-0.9-0.4-0.9

-0.5

-0.3

+0.6+1.1

-0.3-0.2

-0.2

-1.3

+0.1-1.6

εNd (at 1875 Ma) value

22

similar initial 87Sr/86Sr ratios of 0.7030 ±0.0009 and 0.7042 ± 0.0027, respectively.

The overall Sm/Nd of the post-kinematicgranitoids is a valuable tool towards con-straining the age of their protolith (see PaperIII). Utilizing the partial melting model of pa-rental melt mixtures used in for the Jämsä plu-ton (90% ferrodiorite having undergone par-tial melting of 50% + 10% lower crust mate-rial having been partially melted 10%; ex-plained below in the petrogenetic model), thepartial melt model predicts a change in Sm/Ndfrom 0.203 in the protolith to 0.176 in theJämsä pluton, a ∆Sm/Nd of -13% (see Appen-dix 2 in Paper IV for BDC values and traceelement concentrations). Similarly, in thewestern CFGC, the protolith model of theHonkajoki granites (60% ferrodiorite partiallymelted 50% + 40% lower crust material par-tially melted 10%) predicts a shift from Sm/Nd in the protolith 0.201 to 0.168 in the Hon-kajoki granite, ∆Sm/Nd of –16.3%. The aver-age modeled value for the change in Sm/Nd inthe post-kinematic granitoids (∆Sm/Nd = -15%) provides an important boundary condi-tion for the isotopic modeling of the post-kinematic magmatism in relation to the age ofthe source material and the identification ofProterozoic crustal domains in southernFinland (see Paper III).

PETROGENESIS OF THE 1.88–1.87GA POST-KINEMATIC GRANI-TOIDS

ProtolithIn the absence of direct source rock indicators(i.e. magmatic enclaves, cognate xenoliths,autoliths, etc.), determination of source rockcompositions is largely based on geochemicaland isotopic constraints, and trial and errorpartial melt modeling of probable mantle andlower crust rock types. It has been suggestedthat the granites of the 1.88–1.87 Ga post-kinematic suite were produced from a rela-tively anhydrous mafic to intermediate granu-lite from the lower to middle crust and frommafic mantle derived material related to maficintra- and under-plating during the Svecofen-nian orogeny (Paper II). It is apparent frompartial melt modeling of possible granulitic

protoliths that melting of average lower crustcompositions for mafic to intermediate granu-lite, or even felsic granulite (Rudnick & Foun-tain, 1995; Hölttä et al., 2000) as the solesource material, is not suitable to produce aparental melt for the Jämsä or Honkajokisuites. Fig. 13a and 13b show that signifi-cantly different partial melt parameters (>90%partial melting for compatible and <30% forincompatible rare earth elements) are requiredto produce melts from lower crust materialwith rare earth element concentrations thatstart to resemble those observed in Jämsä andHonkajoki. Landenberger & Collins (1996)concluded that differences in composition ofC-type and A-type magmas reflect variation inthe anhydrous mineral assemblages that re-mained in the lower crust after the previous I-type granitoid magmatism. The similar iso-topic compositions across the CFGC (PaperIII) suggest that the source material for thepost-kinematic suite was relatively similar de-spite the variation in pluton character (i.e. C-type character in the east and A-type characterin the west). Considering the similarity inmineralogy and bulk chemical composition ofmany of the post-kinematic granites and manyanorogenic granitoids (cf. the Wolf River ba-tholith, Anderson & Cullers, 1978; theSherman batholith, Frost et al., 1999; thePikes Peak batholith, Smith et al., 1999), asimilar type of source material may be a likelycandidate as the source for the Jämsä andHonkajoki plutons.

Isotope geochemistry suggests that thesource material was characterized by nearchondritic initial Nd isotope values, very lowoverall Rb/Sr (initial 87Sr/86Sr ratios for theJämsä pluton and the Perämaa mafic intrusionat 0.7028 and 0.7032, respectively) and mod-erate U/Pb ratios (see Paper III). However, itwould be unrealistic to assume that a pre-dominantly mantle derived source, generatedat the base of anomalously thick crust, did notincorporate melt fraction from crustal materialduring melting and ascent. Basaltic underplat-ing and intraplating plays an important role incrustal growth and differentiation, and isprobably the cause for the anomalously thickcrust in east-central Finland (Korja et al.,1993; Korja & Heikkinen, 1995). Underplat-ing and intraplating of basaltic magma pro-

23

Figure 13. Chondrite normalized rare earth patterns of possible source rocks in the petrogenesis of the 1.88–1.87 Ga post-kinematic suite. Normalized REE patterns and partialmelting trends are shown for (A and B) two compositions of lower crust xenoliths from east-central Finland, (after Hölttä et al., 2000); (C) ocean island basalt (after Sun &McDonough, 1989); and (D) ferrodiorite (after Mitchell et al., 1996) and compared with calculated averages of biotite-hornblende granite and biotite granite from the Honka-joki complex and pyroxene-bearing quartz monzonite and biotite-hornblende granite from the Jämsä pluton (normalization values after Boynton, 1984). Shaded areas representtrends for partial melting between 90% (dashed line), and 50% (A), 10% (B), and 10% (C and D) (dotted line).

1000

100

10

1

1 0 %

9 0 %

La Ce Nd Sm EuGd Dy Er Yb Lu

O c e a n I s l a n d B a s a l t

P a r t i a l M e l t i n g

1000

100

10

1

1 0 %

9 0 %


F e r r o d i o r i t e

P a r t i a l M e l t i n g

1000

100

10

1

5 0 %

9 0 %


X e n o l i t h 1 - P a r t i a l m e l t i n g

( 7 0 % h b l , 2 3 . 6 % p l g , 6 . 4 % c p x )

1000

100

10

1

3 0 %

9 0 %


X e n o l i t h 2 - P a r t i a l m e l t i n g

( 1 4 . 6 % h b l , 2 7 . 8 % p l g ,

3 8 . 6 % c p x , 1 7 % g t , 2 % b t )

H o n k a j o k i b i o t i t e -

h o r n b l e n d e g r a n i t e

J ä m s ä p y r o x e n e - b e a r i n g

q u a r t z m o n z o n i t e

A)

B) D)

C)

24

vide a heat source to melt granulitic lowercrust and may yield fractionates to producemafic and ultramafic crystal cumulates plusevolved magmas that may intrude the uppercrust or crystallize in the lower crust and un-dergo subsequent melting to produce moresilicic partial melts (Rudnick, 1992).

For the post-kinematic plutons of theCFGC, therefore, geochemical constraintssuggest (1) an Fe-enriched mantle derivedprotolith, and (2) assimilation of crustal mate-rial during melt generation of the graniticmagmas. Geochemical modeling of partialmelting (batch melting) and mixing betweenmantle and crust source rocks help to con-strain rock types from which the post-kinematic suite may have evolved.

BasaltDue to their iron enriched nature and rela-tively reduced redox state, differentiatedtholeiitic basalt has been suggested as a possi-ble protolith for reduced rapakivi granites(Frost & Frost, 1997). Geochemical character-istics similar to those observed in the post-kinematic granitoids (e.g., high Zr, FeOtot/(FeOtot+MgO), and K2O) have been observedin high-K fayalite rhyolites derived from dif-ferentiated tholeiitic basalt (Hildreth et al.,1991), giving the possibility that a similarsource might be a viable candidate also for thepost-kinematic suite. The rare earth elementmodeling suggests that 90% partial melting ofocean island basalt is needed to produce rareearth and trace element patterns that wouldresemble the initial liquid composition for theJämsä and Honkajoki plutons (Figure 13c).Given the Fe-rich nature of fractionatedtholeiitic basalt and tholeiitic trends observedin the post-kinematic suite, a basaltic parentcould be a possible parent material. Similarly,Lahtinen (1994) suggested that the pyroxene-bearing granitoids in the eastern CFGC mighthave been derived from differentiated withinplate basalts, possibly incorporating crustalmaterial during ascent. The high Rb, Zr, K2Oand incompatible rare earth element concen-trations support a within plate character, andrule out a direct low-K island arc basaltsource. Even though most of the post-kinematic granitoids have relatively highFeOtot/MgO, most of the Type 3b plutons con-

taining pyroxene throughout do not show astrong tholeiitic trend, but still retain withinplate trace element features. If the post-kinematic suite was directly derived from abasaltic parent, the degree of fractionation af-ter very high degrees of partial melting wouldhave to be high in order to produce the ob-served geochemical trends.

Diorite, Ferrodiorite, FerromonzodioriteFerrodioritic parental material has been pro-posed as a source rock for certain (reducedtype) anorogenic granites and anorthosite-mangerite-charnockite-granite (AMCG) re-lated intrusions (Frost et al., 1999; Scoates etal., 1996; Mitchell et al., 1996). Ferrodiorite isprobably a more likely candidate than basaltbecause it has a lower melting point than ba-salt, and ferrobasalts and eruptive equivalentsare found in many extensional settings (Frost& Frost, 1997). As Scoates et al. (1996) haveshown, monzonitic rocks with >4.0% K2O atintermediate SiO2 contents (~60%) can beformed from ferrodioritic source composi-tions, but only at very low degrees of partialmelting (~10%). These major element meltcompositions are very similar to the composi-tions of the pyroxene-bearing plutons in theCFGC. However, rare earth element modelingconstraints suggest that the possibility of ob-taining trace element concentrations similar tothose observed in the Jämsä or Honkajoki plu-tons from a solely ferrodioritic source by lessthan 30% partial melting is highly unlikely(Fig. 13d). Rare earth element modeling indi-cates 30–50% partial melting of ferrodioritewould produce compositions similar to aprobable initial liquid for the Jämsä pluton,but trace modeling and major element concen-trations from experimental work require lowerdegrees of partial melting.

Mafic granulite (lower crust)The results from the deep seismic soundingstudy Global Geoscience Transect SVEKA(GGT/SVEKA) across the CFGC indicatesthe presence of anomalously thick, fairly het-erogeneous lower crust (Korsman et al.,1999). The lower crust was delaminated as aresult of mafic underplating and is composedpartly of eclogite and partly of granulites(Korsman et al, 1999; Hölttä et al., 2000).Xenoliths in kimberlite pipes from east-

25

central Finland are the only known examplesof lower crust assemblages from which tomodel lower crust partial melt constituents.Xenolith modal compositions range fromlargely hornblende, plagioclase, and pyroxeneto equal amounts of plagioclase, pyroxene andgarnet, with minor amounts of hornblende andphyllosilicates (cf. Hölttä et al., 2000). It ap-pears very unlikely that the mafic granuliteresidue could be the sole source for the post-kinematic plutons (Figure 13a and 13b). Traceelement and rare earth element models wouldrequire extremely low partial melts of horn-blende-rich granulite, and strongly disfavorgarnet-rich compositions as the sole sourcematerial (Figure 14). The geochemical charac-teristics and intensive parameters suggest arelatively hot and dry source, which does notfavor a hornblende-rich granulite as a sourcematerial for the post-kinematic granitoids. Un-der fluid absent conditions, however, reac-tions of hornblende and quartz to produce py-roxene, plagioclase and melt could occur

(Rushmer, 1991), and would generate suitableprotolith compositions to the post-kinematicgranitoids.

CrystallizationModeling (fractional crystallization or ‘FC’)the effects of individual mineral fractionationon trace element distribution in the Jämsä andHonkajoki plutons show which minerals couldhave been important in magmatic evolution.The Ba vs. Sr plot shows trends consistentwith major fractionation of feldspars (Figure15).

The Jämsä pluton shows fractionation offeldspar, predominantly alkali feldspar in mi-nor proportions from the quartz monzonite tothe biotite-hornblende granite. Further frac-tionation at a considerably higher proportionproduces the quartz-rich granite. This is con-sistent with field observations as the biotite-hornblende granite covers ~70% of the totalpluton area and the quartz-rich granite and

Figure 14. Sr vs. Ba scatterplot diagram of rocks from the Honkajoki and Jämsä plutons, trends for partialmelting of ferrodiorite and xenoliths from east-central Finland, and mixing trends between ferrodiorite partialmelt and xenolith partial melts. The area designated as “Melt Source” area is the best-fit mixing ratio of partialmelts between ferrodiorite and xenolith 1, as deduced from rare earth element patterns. The path of the evolu-tion trend for Sr and Ba is illustrated as (A) the parental melt differentiates/fractionates prior to emplacement,(B) fractionates after emplacement, and (C) differentiates/segregates/fractionates during the later stages of crys-tallization. Jämsä samples are represented by: open circles (pyroxene-bearing margin); open circles with a hori-zontal line (hornblende-biotite granite); open circles with a horizontal and vertical line (quartz rich granite);and Honkajoki samples are represented by: black squares (hornblende-biotite granite) and grey squares (biotitegranite). The bulk distribution coefficient is a partition coefficient calculated for a rock for a specific elementfrom the Nernst partition coefficients of the constituent minerals and weighted according to proportions.

6000500040003000200010000

0

100

200

300

400

500

600

700

Ba (ppm)

Sr(ppm)

Ferrodiorite - partial melt%

D =0.14

D =1.03

Ba

Sr

Xenolith 1 - partial

D =0.35

D =0.76

Ba

Sr

melt %

Xenolith 2 - partial

D =0.27

D =0.60

Ba

Sr

melt %

51030507090

Mixing ratio% ferrodiorite

25

50

75

25

50

75

Melt Source

C

A: Source differentiation/fractionationB: Post-emplacement fractionationC: Late stage differentiation

& fractionation

A

B

26

quartz monzonite only about 17% and 13%,respectively. The segregate sample from thequartz-rich granite phase shows fractionationin the direction of biotite and hornblende inminor proportions (<30%).

The Honkajoki pluton shows similar frac-tionation trends, but at more evolved compo-sitions, where the biotite-hornblende granite issimilar to the biotite-hornblende granite ofJämsä, and the main felsic phase biotite gran-ite is similar to the quartz-rich granite phase atJämsä. The higher proportion of the moreevolved phase in the Honkajoki pluton sug-gests a larger, more fractionated magmasource and produce a wider range in trace ele-ment concentrations.

REE modeling suggests the source materi-als for both pluton suites were mixed propor-

tions of partially melted lower crust and aman t l e -de r ived fer rod ior i t i c / fe r ro -monzodioritic rock. Figure 16 illustrates mix-ing proportions for partial melts of ferrodioriteand lower crust material. The xenolith compo-sitions (that best represent the range of rocktypes in the lower crust in central Finland; seeHölttä et al., 2000) show that hornblende-plagioclase dominated rocks are more suitablesource components than garnet-bearing pyrox-ene-dominated compositions. The mixingmodel also shows that a source melt of mainlyferrodiorite composition will increase in Baand slightly decrease in Sr during early frac-tionation of plagioclase and clinopyroxene.After emplacement and fractionation proc-esses, hydration and cooling begin to effectthe crystallizing phases (at lower pressure)and Sr decreases with decreasing Ba. The vec-tor created between the phases of each pluton

Figure 15. Ba vs. Sr scatterplot diagram for rocks from the Honkajoki and Jämsä plutons, and trends for indi-vidual mineral fractionation in the evolution of each suite. The presumed parental melt for the Honkajoki plu-ton designated with the letter ‘H’ and the composition of the residual liquid after fractionation of the main min-eral phases observed in the biotite-hornblende granite (10% hornblende, 4% biotite, 46% alkali feldspar, 39%plagioclase) shown at 10% increments. The path originating at the “star” symbol represents the presumed pa-rental melt for the Jämsä pluton and illustrates the fractionation of plagioclase and clinopyroxene at crystalliza-tion increments of 10% prior to the emplacement of the Jämsä initial liquid. The beginning of crystallization inthe Jämsä pluton is designated with the letter ‘J’ and the composition of the residual liquid after fractionation ofthe main mineral phases observed in the biotite-hornblende granite of the Jämsä pluton (10% hornblende, 4%biotite, 46% alkali feldspar, 39% plagioclase) is shown at 10% increments. The sample representing the maficsegregate of the late stage quartz-rich granite in the Jämsä pluton falls near the fractionation trend for 40%hornblende, 45% biotite and 15% alkali feldspar (given in 10% increments) from the late stage quartz-richgranite liquid. Symbols are the same as is indicated in Figure 14.

60050040030020010000

500

1000

1500

2000

2500

3000CpxPlag

Kfsp Bt

HblOpx

Sr (ppm)

Ba(ppm) H

Mafic silicate segregationD =2.56, D =0.25

Ba Sr

Jämsä melt sourcefractionationD =0.25

D =1.68

Ba

Sr

J

Jämsä & HonkajokicrystallizationD =3.21

D =2.89

Ba

Sr

27

Fig

ure

16.C

hond

rite

norm

aliz

edra

reea

rth

elem

ent

patt

erns

illus

trat

ing

the

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984)

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1000

100

10 1

90%:50% ferrodiorite melt +

10%: 10% lower crust melt

Initial Jämsä liquid

(reintergration of all phases)



La

Ce

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu

Initial Jämsä Liquid

1000

100

10 1

10% fractionation

30% fractionation

Average quartz monzonite

Post stage 1 Jämsä liquid

(reintergration of all phases

except quartz monzonite)

La

Ce

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu

Stage 1 fractionation

1000

100

10 1

70% fractionation

50% fractionation


(quartz-rich granite & mafic

segregate reintergration)


(quartz-rich granite & mafic

segregate reintergration)

Average biotite hornblende granite

La

Ce

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu


1000

100

10 1

30% fractionation

Mafic segregate

10% fractionation

Average quartz-rich granite

La

Ce

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu


A)

B)

C)

D)

28

suite reflects back to the initial proportion ofpartial melt involved in producing the initialmelt composition/trace element concentra-tions. In Figure 14, it is shown that a maficgranulite of pyroxene dominant modal com-position probably had little involvement inproducing the initial melt for Honkajoki orJämsä. The initial melt is constrained to com-positions of higher Sr and Ba. Furthermore,the vector created between phases of eachsuite suggests that the Honkajoki initial meltformed at higher partial melt proportions inthe ferrodiorite/mafic granulite mixture modelthan that of Jämsä. The area designated“source melt” in Figure 14 covers the range ofsource-melt mixtures derived from REE mod-eling for both pluton suites.

The initial Jämsä melt was calculatedfrom a reintegration of the phase compositionsby aerial distribution. The partial melt mixingmodel of 80–90% ferrodiorite at 30–50% par-tial melt and 10–20% mafic granulite partiallymelted at 10–30% creates a fairly close ap-proximation of the initial Jämsä liquid compo-sition (Figure 16a). The fractional crystalliza-tion of the marginal quartz monzonite (Stage1 fractionation) at 10–30% produces a REEpattern close to the approximated melt, or thecomposition of the melt after the crystalliza-tion of the marginal pyroxene-bearing quartzmonzonite (Figure 16b). The crystallization ofthe biotite-hornblende granite (~70% of theentire pluton area) fractionated from the post-Stage 1 liquid at about 70% produces a closeapproximation to the quartz-rich granite liquid(Figure 16c). The quartz-rich granite phase ishost to numerous mafic glomerocrystic pock-ets or pipes that separated from the felsic liq-uid and can be modeled as a third and finalstage of fractionation producing patterns vir-tually identical to the quartz-rich granite hav-ing fractionated about 10-30% of the mineralsand proportions that compose the mafic segre-gates (Figure 16d).

The Honkajoki granites may have beenderived from similar source rocks as theJämsä pluton. In contrast to the in situ closedsystem fractionation model derived for theJämsä intrusion, the Honkajoki complex re-sembles a more open system where the bio-tite-hornblende granite and biotite granite

Figure 17. Chondrite normalized rare earth elementpatterns illustrating the petrogenesis of the Honka-joki pluton (normalization values after Boynton,1984). The initial parental liquid was probably de-rived from (a) 60–70%: 50–70% ferrodiorite partialmelt and 30–40%: ~10% lower crust partial melt.The melt (b) fractionated biotite-hornblende graniteat 30-50%, and (c) the main phase biotite granite at50–70% fractionation.

1000

100

10

1La Ce Nd Sm EuGd Dy Er Yb Lu

Initial Honkajoki liquid



Average biotite granite

Average biotite-

hornblende granite



100

10

1La Ce Nd Sm EuGd Dy Er Yb Lu

30% fractionation


Average biotite-

hornblende granite

50% fractionation

1000Stage 1 fractionation


Post stage 1

Honkajoki liquid


30% fractionation

50% fractionation

1000Stage 2 fractionation

100

10

1

70% fractionation

A)

B)

C)

29

were generated at depth and emplaced asseparate intrusive phases. The initial liquid forthe granite phases is modeled as a simple par-tial melt of ferrodiorite and mafic granulite at60–70% ferrodiorite partially melted at 50–70% mixed with 30–40% mafic granulite par-tially melted at ~10% (Figure 17a). Simplefractionation of the initial melt at 30–50%(Stage 1 fractionation) produces patternsclosely resembling the biotite-hornblendegranite REE pattern (Figure 17b). Furtherfractionation at 50–70% of the post-Stage 1liquid produces patterns similar to the biotitegranite REE pattern (Figure 17c).

The 1.88-1.87 Ga post-kinematic plutonswere formed over a relatively large area andhave phases that differ in mineralogical andgeochemical compositions. The petrogeneticmodel described here, in conjunction with iso-topic evidence, shows that the Jämsä andHonkajoki plutons could have been derivedfrom partial melts of similar source rocks andmay have shared a similar magmatic evolu-tion.

DISCUSSION

The structure of the crust in centralFinlandA detailed geophysical profile across theCFGC shows the depths to the mantle andlower-most high velocity lower crust and isextrapolated into a three dimensional perspec-tive in Figure 18. The depth of the lower crustin the western CFGC near Honkajoki is ~21km, and ~23 km in the eastern CFGC nearJämsä. The depth to the Moho in the westernCFGC is ~54 km in the western CFGC nearHonkajoki, and ~58 km in the eastern CFGCnear Jämsä. The greater depth and thickerlower crust attribute to the character and rela-tive number of Type 3 plutons located in theeastern CFGC. These pyroxene-bearing as-semblages reflect a higher pressure and moreanhydrous nature of the source material gener-ated at greater depths and are probably com-posed of a larger proportion of mantle derivedmaterial. In the eastern CFGC, the deepermantle and thicker lower crust produced morepartial melt of lower crust but contributed alower proportion of crustal melt to the overall

Figure 18. A three-dimensional representation of thedepth to the lower-most high velocity layer in thecrust and the depth to the Moho across the CFGCarea in central Finland (based on data presented byKorsman et al., 1999).

262524

232221

2019

18

17

16

15

25

24

23

22

19

20 21

21

20

19

18

49

50

51

52535455565758596061

53

54

55

56

57

58

58

59

60

57

Dep

thto

the

Low

erC

rust

(km

)

Dep

thto

the

Moh

o(k

m)

CF

GC

30

post-kinematic granitoid magmas. In the west-ern CFGC, the mantle is relatively more shal-low and the lower crust relatively thinner, andthe post-kinematic magmas incorporated ahigher proportion of crustal melt at lower de-grees of partial melt probably due to lowerpressures and a more open system in whichascent was relatively more rapid (i.e. exten-sional tectonic setting). Overall, assuming thatthe present structure reflects that of ~1.88–1.87 Ga, the crustal structure across the CFGCis perhaps an important factor in controllingthe petrogenesis of the post-kinematic gran-ites; the depth to mantle and thickness to ofthe lower crust being a major influence onmineralogical compositions and geochemicalcharacter of the post-kinematic plutons.

The effect of water activity and cooling rateon magmatic evolutionPyroxene-bearing plutons (previously referredto as being derived from C-type magmas inPaper II) predominate in the eastern CFGCand high silica, predominantly biotite-bearingplutons (those with A-type characteristics) aregenerally located in the western CFGC (cf.Paper II). The initial geochemical variation,prior to fractionation processes, is seen as afunction of variation in melt percentage andmixing proportion between melts from lowercrustal material and the mantle derived mate-rial in the lower crust. The variation in min-eral composition is a function of water activ-ity and rate of cooling as the magmas wereemplaced at shallower levels.

Experimental studies on the effect of wa-ter content in granitic systems during ascentshow that water content in the melt fractionincreases dramatically during fractionationwith increased cooling rate when compared toadiabatic systems or systems where the cool-ing rate is lower (cf. Holtz & Johannes, 1994;Johannes & Holtz, 1996). The experimentalapproach can be utilized to explain the obser-vation of anhydrous assemblages (i.e. pyrox-ene and olivine) where heat exchange wasmuch lower during crystallization and morehydrous assemblages (i.e. amphibole and bio-tite, or biotite only) formed due to the increas-ing water content in the melt fraction at an in-creased cooling rate. The emplacement of thepost-kinematic magmas in the eastern CFGC

shortly followed the peak of the orogeny,whereas the plutons in the western CFGCwere emplaced ~10 Ma later when the crustwas relatively cooler. Calculated trajectorypaths for the ascent of granitic melts at differ-ent cooling rates (Holtz & Johannes, 1994)can be applied to the petrological variationobserved in the post-kinematic suite. In Figure19, we assume that two plutons originate fromsimilar a source composition at the same tem-peratures, pressure and water content. Trajec-tory ‘A’ represents the crystallization path ofone pluton at –5 J g-1 kbar-1 and path ‘B’represents the same magma crystallizing at aslightly faster cooling rate, -10 J g-1 kbar-1. Ifwe assume an initial crystal/melt ratio of 50%,and the initial water content (as indicated inFigure 19) of 2.0% in the melt, the model pre-dicts increasing wt.% H2O during the evolu-tion of the magma. Any fractionation proc-esses, assuming a system closed to additionalmagma, could increase the wt.% H2O in theresidual melt dramatically (cf. Holtz & Johan-nes, 1994).

Similar to water content, the ascent andcooling processes of granitic magmas can bestrongly influenced by viscosity. Model calcu-lations show that viscosity of granitic melts isstrongly influenced by water content. Johan-nes & Holtz (1996) show that even though amagma at the same pressure is in a muchcooler environment (200° C less), the viscos-ity is lower at higher water contents. If we ap-ply this to the cooling during ascent model(Figure 19), where increased cooling rate in-creases water content in the residual melt, theviscosity can decrease, promoting fractiona-tion. Viscosities of granitic magmas remainapproximately constant (or increase slightly)during ascent under adibiatic conditions. Con-verse to increased cooling, adibiatic ascentcan produce decompressional melting, de-creasing the wt.% H2O in the residual melt,and increasing the viscosity of the magma. Insuch a case, it is quite possible that the magmacan reach its rheologically critical melt per-centage (typically between 25% and 40%),where it becomes too rigid to behave as amagma and discontinues ascent. Furthermore,many of the plutons in the western CFGCcontain accessory fluorite. Experimental stud-ies on the effect of F on granitic systems show

31

an increase in H2O solubility and lowering ofthe liquidus temperature with increasing Fcontent (cf. Bailey, 1978; Manning, 1981; Jo-hannes & Holtz, 1996). This would allowmagma with a higher water activity to migrateto comparable emplacement levels as graniticmagmas with lower water activity and lower Fcontent.

Temporal shift in magmatism across theCFGCThe geographic position of U-Pb ages for thepost-kinematic granitoids show a trend of de-creasing ages from northeast to west-southwest across the CFGC (see Paper III).The plutons in the northeast have ages around1885 Ma and those in the in the west-southwest have ages between 1870–1875 Ma.The shift is accompanied by a change in char-acter of the post-kinematic granitoid magma-tism: the older plutons are generally lessevolved pyroxene-bearing (charnockitic)granitoids, and the younger intrusions havemany characteristics usually associated withA-type granitoids. The thickness of the conti-nental crust across the CFGC region shows a

significant change from east to west where thehigh velocity lower crust is ~7 km thicker inthe east than in the west (Korsman et al.,1999). The intial accretion of the centralFinland arc to the Archean craton producedfractures or conduits, even though the crustmay have been more plastic at the time, forwhich the early post-kinematic granites coulduse to migrate through the crust. The accretionof the southern Finland arc occurred shortlyafter, about 10 – 30 Ma, which induced stressin the southern portion of the central Finlandarc. The temporal distribution of post-kinematic plutons may not, necasarily, be as-sociated with separate basaltic underplating/intraplating “events”, but rather the migrationof tectonic stress that induces fractures or con-duits for mafic magmas to induce melting, andallows granitic magmas to reach their currentemplacement level.

ACKNOWLEDGEMENTS

My personal thanks to Bo Johanson and LassiPakkanen for their invaluable microprobe as-

Figure 19. Pressure–Temperature diagram showing the liquidus curves for the haplogranite system and mini-mum melt compositions and specified water contents (after Johannes & Holtz, 1996). Pressure–Temperaturepaths (A) and (B) represent initially identical granitic magmas ascending at different cooling rates (-10 J g-1

kbar-1 and –5 J g-1 kbar-1, respectively). Thermobarometry calculations for the Jämsä pluton and Honkajokicomplex mineral equilibria are shown for comparison.

Initial wt.% H O = 2.02

10

9

8

7

6

5

4

3

2

1

0

600 700 800 900 1000

Increasing cooling rate

& water content during

ascent:

(A) 10 J/g / kbar

(B) 5 J/g / kbar

A

B

Water saturated solidus

Jämsäamphibole

JämsäQUIlFP

ress

ure

(kb

ar)

Temperature (°C)

4.0

3.0

1

.0

9.0

8.0

7.0

6.0

5.0

Honkajokiamphibole

Dry solidus

32

sistance at the Geological Survey of Finland.Lars Rämö is acknowledged for providing hisexpertise in producing high quality thin sec-tions for petrography and microprobe studies.The isotope laboratory personel at GTK, espe-cially Matti Vaasjoki, are acknowledged fortheir assistance in acquiring the zircon sam-ples needed for stable and radiogenic isotopestudies and providing the U-Pb, Sm-Nd, Pb-Pb, and Sr-Rb isotope data used in this study.I am indebted to Ilmari Haapala for all his as-sistance and effort in organizing and support-ing the project from the moment I began mystudies at the University of Helsinki; MikkoNironen for his assistance and guidance at theGeological Survey of Finland and in the field;and Tapani Rämö for his invaluable assistancewith isotopic aspects of the study, the numer-ous discussions on the geochemical and petro-genetical aspects of granite petrology, hismentorship and efforts to support me and myresearch endeavors. Comments and sugges-tions from Anders Lindh and Raimo Lahtinenwere greatly appreciated in the final prepara-tion of the thesis summary. Most of all, I’mthankful for the patience and support of mywife, Virva, and my children, Eeli and Otso,during the years it has taken to complete thiswork. The research has been funded by theAcademy of Finland (Project 36002) and theUniversity of Helsinki ‘Grant for finishingdoctoral dissertations’, and is a contribution toIGCP Project 426 (Granite Systems and Pro-terozoic Lithospheric Processes).

REFERENCES

Andersen, D.J., Lindsley, D.H., & Davidson,P.M. (1993). QUIlF: A Pascal pro-gram to assess equilibria among Fe-Mg-Mn-Ti oxides, pyroxenes, olivine,and quartz. Computers and Geo-sciences 19 (9), 1333–1350.

Anderson, J.L. & Cullers, R.L. (1978). Geo-chemistry and evolution of the WolfRiver batholith, a late Precambrianrapakivi massif in north Wisconsin,U.S.A. Precambrian Research 7,287–324.

Bailey, D.K. (1978). Continental rifting and

mantle degassing. In: Neumann, E.R.& Ramberg, I.B. (eds.), Petrology andgeochemistry of continental rifts.Dordrecht: D. Reidel, 1–13.

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the petrogenesis of the 1.88–1.87 ga post- kinematic

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